Funky Tach Issue

I've been refitting my '86 13 foot whaler with a 1984 Evinrude 30 and I wanted to add a tach to the console. I picked up a Teleflex Lido reading up to 7k RPM and I have since hooked it up and tested it out. The tach reads seemingly accurate up to about 4200 - 4400 RPM. What happens when I push the trottle a bit more is the motor revs higher but the tach starts dropping down? Anyone have an idea what might cause this? :huh

Hey guys, thanks for the responses. The pole setting is set on 5 - (10 pulses from alternator per revolution/2 = 5). This is why I'm kind of scratching my head... It's a little difficult to try turning the dial setting at 25MPH, but at this point I think it's the only thing left to do before digging further. I would imagine that setting it at 5 should be all it takes, right?